This invention relates to x-ray diagnostic systems and more particularly to a system which concentrates x-rays from a source and delivers them to an x-ray spectrometer.
Most focusing x-ray optics take advantage of total reflection at glancing angles of incidence. Total reflection occurs only when the angle of incidence is less than a critical angle that depends upon the properties of the reflecting material and the x-ray energy. Although prior art designs may vary according to application, most such designs have used metal or glass substrates with coatings of nickel, gold or iridium at glancing angles ranging from 10 to 150 arc minutes. Double-reflection geometries of the Wolter-I or Kirkpatrick-Baez types have been developed to focus a parallel beam of x-rays. The Wolter-I configuration consists of confocal parabaloid-hyperboloid shells and has been used most often for x-ray telescopes designed for high angular resolution. This optic is axially compact, has a moderate field of view and, in some cases, a large number of telescopes can be nested to fill a substantial fraction of the available entrance aperture. An approximation to the Wolter-I design replaces the precisely figured optics with simple cones. Telescopes based upon this approximation have been developed for various astrophysical payloads. The Kirkpatrick-Baez geometry uses two parabolic surfaces for parallel-to-point focusing, and it has been adapted to point-to-point geometries for x-ray microscopes. Recently, optics based upon bundles of glass capillary tubes have emerged as a method for focusing x-rays. The x-rays undergo numerous reflections as they travel through the glass channels causing these optics to have lower efficiency than the double reflection systems referred to above.
Electron microscopes are widely used in many applications including in the semiconductor fabrication industry. When targets are irradiated with electrons, x-rays are generated as a side effect. The x-ray spectrum provides information about elements contained in the target so that x-rays are often detected for analysis. In the prior art, it is known to place a detector such as a lithium-drifted silicon or germanium detector very close to the target in a scanning electron microscope. Such detectors are typically mounted on the end of a cold finger cooled by thermal conduction by means of a quantity of liquid nitrogen which boils at 77 kelvin. Higher resolution can be achieved utilizing detectors cooled to approximately 0.1 kelvin and in this context it may be desirable to locate the detector outside of the SEM enclosure. However, because of the well known square law dependence of intensity on distance from a source of x-rays, as a detector is moved farther from the source, the intensity drops which degrades the performance of a spectrometer receiving the x-rays. It is also known to use monolithic polycapillary glass optics within an SEM enclosure to concentrate x-rays for subsequent analysis but not to use any such concentrator beyond the confines of the SEM enclosure.